Space Technology: Sun, observatories and the solar system

Aditya-L1 and space observatories

Aditya-L1 is the first space-based Indian mission to study the Sun. Launched on PSLV-C57, it was placed in a halo orbit around Lagrange point L1 on 6 January 2024. L1 is a balance point about 1.5 million km from the Earth where the pulls of the Sun and Earth even out, so a spacecraft can park there. From L1 it has a continuous view of the Sun, never blocked by Earth. It studies the photosphere (visible surface), chromosphere and corona (outer atmosphere). Four payloads view the Sun directly and three measure particles and fields on the spot (ISRO, Aditya-L1 page).

Watching the Sun helps forecast space weather, much like a weather office warns of storms. On NOAA’s scale, an extreme geomagnetic storm can cause grid blackouts and transformer damage, degrade satellite navigation and disturb spacecraft (NOAA Space Weather Prediction Center). If a major solar storm (solar flare) reaches the Earth, GPS and power grids can fail, intense auroras appear, satellite orbits are disturbed and shortwave radio over the poles is cut. There are no tsunamis or forest fires (Prelims 2022: answer (c), 1, 3, 4, 6 and 7 only).

AstroSat, launched on PSLV-C30 on 28 September 2015, is India’s first dedicated astronomy mission. It observes in X-ray, optical and ultraviolet bands. It weighs 1,515 kg and orbits at 650 km, inclined at 6 degrees (ISRO, AstroSat page). So the claim of a 2000 kg satellite at 1650 km is wrong. UPSC’s key also rejects the claim that only the USA and Russia had launched a similar observatory before (Prelims 2016: answer (d), Neither 1 nor 2).

The James Webb Space Telescope (JWST), launched on 25 December 2021, orbits the Sun at Lagrange point L2, 1.5 million km from Earth, behind a five-layer sunshield the size of a tennis court. Hubble, by contrast, orbits Earth at about 560 km (NASA/ESA Webb). Mains 2022 asked what makes JWST better than earlier space telescopes. Points:

• a much bigger mirror,
• infrared vision to see the earliest galaxies and inside dust clouds, much like night-vision goggles,
• a cold, stable position at L2,
• goals of studying the first stars, galaxy formation and exoplanet atmospheres.

Planetary missions, RTGs and gravitational waves

Some key planetary missions (NASA):

• MESSENGER was the first spacecraft to orbit Mercury.
• Cassini studied the Saturn system, not Venus, and ended with its grand finale on 15 September 2017.
• Voyager 1 and 2, launched in 1977, toured the outer planets and reached the interstellar boundary, the edge of the Sun’s bubble, in 2012 (Voyager 1) and 2018 (Voyager 2).

So Messenger was for mapping and investigating Mercury, and Voyager explored the outer solar system (Prelims 2014: answer (b), 2 and 3 only).

Far from the Sun, solar panels give too little power. NASA’s radioisotope power systems (RTGs) turn heat from the natural decay of plutonium-238 into electricity (NASA). Think of a hot water bottle that stays warm for decades. They are not fission reactors, so statement 1 is false. They power the onboard systems of spacecraft and use plutonium-238 (Prelims 2024: answer (b), 2 and 3 only).

LISA (Laser Interferometer Space Antenna), led by ESA, will be the first space-based gravitational wave observatory. It will use three spacecraft linked by six laser beams, with launch planned for 2035 (ESA, a target). Its earlier name was evolved LISA (eLISA). So its purpose is to detect gravitational waves (Prelims 2017: answer (b)), and a trio of spacecraft flying in formation with lasers between them is Evolved LISA (Prelims 2020: answer (d)).

Source: NASA and ESA mission pages.

Asteroids, comets, exoplanets and stars

Asteroids are rocky leftovers from the formation of the solar system, mostly found between Mars and Jupiter (NASA). Comets are cosmic snowballs of frozen gases, rock and dust. Near the Sun they heat up into a glowing head with a tail. They come from the Kuiper Belt and the Oort Cloud, far beyond Neptune (NASA). So asteroids are small rocky planetoids and comets show a tail (true), but comets are not found between Venus and Mercury (Prelims 2011: answer (b), 1 and 3 only).

NASA’s DART was the first mission to show asteroid deflection by kinetic impact, which means pushing an asteroid off course by crashing into it, like nudging a ball with another ball. Launched on 24 November 2021, it hit Dimorphos, a 160 m moonlet of the asteroid Didymos, on 26 September 2022 (NASA). Mains 2024 asked what asteroids are and what strategies exist to prevent an impact catastrophe. Points: survey and track near-Earth objects, kinetic impactors like DART, and international planetary defence planning.

The habitable zone is the range of distances from a star where liquid water could exist on a planet’s surface. NASA notes these are also called Goldilocks zones: not too hot, not too cold, just right, as in the fairy tale (NASA). So the term relates to the search for Earth-like planets in outer space (Prelims 2015: answer (c)).

Some star terms (NASA):

• Cepheid variables are stars whose brightness changes in a regular cycle, which astronomers use to measure distances.
• A nebula is a cloud of dust and gas.
• Neutron stars form when a massive star runs out of fuel and collapses.
• Pulsars are neutron stars whose beams sweep past us as pulses, like a lighthouse.

In the 2023 pairs, Cepheids and Nebulae were swapped. Only neutron stars formed when massive stars run out of fuel and collapse matches (Prelims 2023: answer (a), Only one).

What UPSC asks: Prelims: Aditya at L1, Messenger to Mercury, asteroids between Mars and Jupiter. Mains: storms hit GPS and grids, AstroSat 1,515 kg, RTGs run on Pu-238 decay, LISA for gravity waves, Goldilocks means habitable, pulsars are neutron stars.